: Understanding how human embryonic stem (ES) cells can proliferate without limit and yet retain the ability to differentiate to any cell type is the theme that links the three individual projects of this proposal. Project 1 will identify novel histone modifications in human ES cells using a new mass spectrometry technique that allows an unprecedented ability to identify and map posttranslational protein modifications. Histone modifications in human ES cells will be identified globally, at promoters, and at select genes directly regulated by critical pluripotency factors. We will also examine the role of histone H3 variants in establishing long-term epigenetic memory. These studies will determine whether there is a novel histone code in pluripotent cells and determine how histone modifications change dynamically during differentiation. Project 2 examines the critical events that occur in the window of time during which human ES cells commit eprogram differentiated cells to a pluripotent state. We have previously reported that when myeloid cells are fused with human ES cells, the myeloid nucleus is reprogrammed to an ES cell state, indicating that transacting factors in ES cells are sufficient to mediate nuclear reprogramming. Our preliminary results suggest that over expressing combinations of human ES cell-enriched genes can reprogram myeloid cells, and this project will optimize this reprogramming. The combination of these projects will provide an increased understanding of the pluripotent state and the basic processes by which a cell can leave or return to that state. Such an understanding will be important to transplantation and regenerative medicine. Lay Description: Human ES cells are special because they can grow without limit and can give rise to all other cell types. Here we will try to understand why human ES cells have this remarkable developmental potential, and develop conditions to convert a cell with a more limited potential to an ES cell. Such reprogramming has implications for transplantation and regenerative medicine. 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